Correlation between stimulation strength and onset time of signal traveling within the neocortical neural circuits under caffeine application

In general, strength of input to neocortical neural circuits affects the amplitude of postsynaptic potentials (PSPs), thereby modulating the way signals are transmitted within the circuits. Caffeine is one of the pharmacological agents able to modulate synaptic activities. The present study investigated how strength of input affects signal propagation in neocortical circuits under the application of caffeine. Spatio-temporal neural activities were observed from visual cortical slices of rats using optical recording methods with voltage-sensitive dye. Electrical stimulations were applied to white matter in the primary visual cortex with bath-application of caffeine. When the strength of stimulation was 0.3 mA, signals propagated from the site of stimulation in the primary visual cortex toward the secondary visual cortex along vertical and horizontal pathways. When stimulation strength was reduced from 0.3 mA to 0.07 mA, start of signal propagation was delayed about 25 ms without affecting field PSP amplitude or the manner of signal propagation. Conversely, co-application of caffeine and d-2-amino-5-phosphonovaleric acid (d-AP5) did not induce delays in signal start. These findings suggest that conversion of neural code from amplitude code to temporal code is inducible at the level of neocortical circuits in an N-methyl-d-aspartate (NMDA) receptor activity-dependent manner.

► We investigated how strength of input affects behaviors of output signals.
► Caffeine enhanced neural activities in the rat brain slice of visual cortex.
► When the stimulation strength was reduced, start of signal traveling was delayed.
► Visual cortex has the ability to translate amplitude code into temporal code.
► NMDA receptor activation is concerned with the transformation of coding.